Add rewind on pause

[skorokithakis]

Moves the position back when a book is continued, proportionally to how long that book was paused:

rewind = amount * min(pause, 300s) / 300s

So a momentary pause moves almost nothing, and a pause of five minutes or more moves the full amount.

The setting

"Rewind on pause" in Settings -> Playback -> Seek settings: a toggle and an amount (1-60s, presets 5/10/15/30/60), on by default with 30s. It is part of the settings backup, and the amount is clamped to 1-60 both on read and on import, so an edited backup cannot produce a strange rewind.

This puts the dormant RewindOnPauseTime domain class to use and removes the SeekTimeOption enum, which nothing else referenced.

Per book, not global

The last active time is stored per book (a bookId -> epoch millis map, capped at the 100 newest books). Pausing book A and starting book B must not reset A's clock: the rewind models how much of that book the listener forgot. A book with no stored time gets the full rewind, because "no record" means the listener probably forgot more, not less.

One choke point

The rewind is applied by a thin ForwardingPlayer around the ExoPlayer that the MediaLibrarySession is built on. The seek therefore happens before the audio starts, so there is no audible jump, and every source behaves the same: the app, the notification, media buttons, Android Auto, the widget, the sleep timer, a book switch, and a resume after the process was killed.

Two details in there:

• It fires only on a real playWhenReady false -> true change, so a repeated play command during a session cannot rewind.
• The automatic resume after a short audio focus loss (a notification, a navigation prompt, a call, with the "pause" policy) never issues a play command, because ExoPlayer only suppresses playback. That path is handled from the playback suppression reason instead. A buffering stall also stops playback and is deliberately excluded, because nothing was missed. On this one path the audio has already restarted when the seek happens, so a very short blip is possible; avoiding it would need a stop/seek/start dance that risks losing the audio focus.

Notes for review

• Chapter lengths come from the book, not from the player timeline: the timeline reports C.TIME_UNSET for chapters that ExoPlayer has not prepared yet, which would silently disable the rewind on the first play after opening the app.
• The book that the player has loaded is tracked in the handler rather than read from the playing item preference. On a book switch the preference already points at the next book before the previous one stops, so reading it in the listener would record the stop against the wrong book.

Tests

Unit tests for the proportional maths and the seek target (including crossing chapter starts and clamping at the start of the book), the preference storage and its cap, the handler (book switch, suppression resume, the playWhenReady gate), the settings backup, and the view model. ./gradlew lintKotlin test passes.

[skorokithakis]

Superseded by GrakovNe#492, which targets upstream.